The present invention relates to impingement cooling for a shroud assembly surrounding the rotating components in the hot gas path of a gas turbine, and particularly relates to supplying purge air to the gaps between the inner shroud segments to cool the segments and to prevent hot gas ingestion into the gaps.
Shrouds employed in a gas turbine surround and in part define the hot gas path through the turbine. Shrouds are typically characterized by a plurality of circumferentially extending shroud segments arranged about the hot gas path, with each segment including discrete inner and outer shroud bodies. Conventionally, there are two or three inner shroud bodies for each outer shroud body, with the outer shroud bodies being secured by dovetail-type connections to the stationary inner shell of the turbine and the inner shroud bodies being secured by similar dovetail connections to the outer shroud bodies. The inner shroud segments directly surround the rotating parts of the turbine, i.e., the rotor wheels carrying rows of buckets or blades. Because the inner shroud segments are exposed to hot combustion gases in the hot gas path, systems for cooling the inner shroud segments are oftentimes necessary to reduce the temperature of the segments. This is especially true for inner shroud segments in the first and second stages of a turbine that are exposed to very high temperatures of the combustion gases due to their close proximity to the turbine combustors. Heat transfer coefficients are also very high due to rotation of the turbine buckets or blades. To cool the shrouds, typically relatively cold air from the turbine compressor is supplied via convection cooling holes that extend through the segments and into the gaps between the segments to cool the sides of the segments and to prevent hot path gas ingestion into the gaps. The area that is purged and cooled by the flow from a single cooling hole is small, however, because the velocity of the cooling air exiting the cooling hole is high, and the cooling air diffuses like a jet and flows into the hot gas flow path.
Previous design methods thus required multiple cooling holes in close proximity to each other, using increased amounts of cooling air from the compressor (and additional machining) which, in turn, reduces the efficiency of the turbine.
In an exemplary embodiment of the invention, a cooling circuit for purging cooling air into the gaps between inner shroud segments includes convection holes that incorporate diffusers at their respective outlet ends. Each diffuser may include an elongated, substantially rectangularly-shaped outlet recess or cavity with a cross-section that tapers away from (i.e., increases outwardly from) the respective convection hole, terminating at the face of the segment. More specifically, the convection hole extends at an angle of about 45xc2x0 relative to the segment face, opening into the diffuser recess near a rearward or upstream end of the recess, relative to the direction of purge or cooling flow. The diffuser recess includes a long tapered portion extending in the flow direction (or forward of the convection hole) and a short tapered portion extending in a direction opposite the flow direction. The end result is that the cooling or purge air begins to diffuse before it reaches the face of the segment, enhancing the cooling of the segment edges. While the cooling or purge air does lose some velocity in the diffuser, sufficient pressure is maintained to prevent hot gas path gases from entering the gaps between the inner shroud segments.
Accordingly, in its broader aspects, the invention relates to an inner shroud assembly for a turbine comprising a plurality of part-annular segments combining to form an inner, annular shroud adapted to surround rotating components of a turbine, each segment having a pair of end faces that are juxtaposed similar end faces on adjacent segments with gaps therebetween; at least one convection cooling hole in the part segment, opening along at least one of the pair of end faces; said at least one cooling hole opening into a diffuser recess formed in one of the pair of end faces for diffusing the flow of cooling air into the gap.
In another aspect, the invention relates to a segment for a turbine shroud assembly comprising a segment body having a sealing face and opposite end faces; and at least one convection cooling hole extending through the segment body and opening into a diffuser recess formed in a respective end face of the segment body.
In still another aspect, the invention relates to a method of purging cooling air into gaps between adjacent part annular segments in a turbine shroud assembly comprising a) supplying cooling air through one or more cooling holes formed in each segment, each cooling hole opening along an end face of the segment; and b) diffusing the cooling air before it reaches the end face of each segment.